1. Field of the Invention
The present invention relates to a method and apparatus for sensing brushless DC average motor current, and more particularly to a method and apparatus which sense average motor current from over current protection circuitry.
2. Related Art
In the design of brushless DC motor drive systems, there is often a requirement to sense the motor average current. Effectively, the motor average current is the average current being drawn from the input power supply by the motor drive system. In brushless DC motor drive systems that are employed for safety-critical applications, such as but not restricted to electro-hydraulic power steering systems, knowledge of the motor average current can be used by the microcontroller for protecting the motor. For example, the microcontroller can control the motor such as to limit its average current to a specific maximum value. This maximum value can be derived based on the maximum current rating of the motor.
FIG. 1 shows a typical apparatus used for sensing the average motor current in a brushless DC motor drive system 10.
In FIG. 1, the dc-link current idc is measured using a current sensor 12. For example, this current sensor can be a dc-link shunt resistor, a Hall effect current sensor, etc. An amplifier and filter stage 30, with a very heavy filter (e.g. 0-10 Hz bandwidth), is used to amplify and filter the dc-link current and obtain the motor average current. The resulting motor average current signal can then be sampled by the microcontroller 20 and used for protecting the motor.
In brushless DC motor drive systems that are employed for safety-critical applications, such as but not restricted to electro-hydraulic power steering systems, there is often an additional requirement to sense the instantaneous dc-link current and use it for protecting the system power components against over current conditions. A typical circuit used to detect instantaneous over current conditions is also shown in FIG. 1. In this case, an amplifier and filter stage 14 with a light filter (e.g. 0-20 KHz bandwidth) appropriately conditions the measured dc-link current. The output of the amplifier and filter stage is a pulsating voltage that represents the instantaneous dc-link current. This voltage is then presented to multiple comparators 16a, 16b, . . . , 16n. Each comparator compares the instantaneous dc-link current to a specific threshold that represents an instantaneous current level deemed more or less dangerous. If the output of a comparator switches, then the microcontroller immediately becomes advised of an over current condition. In such case, as a means for protection, the microcontroller turns off all inverter switches for the remainder of the PWM cycle thereby effectively reducing the PWM duty cycle.
Thus, typically, two amplifier and filter stages 14 and 30 are used to condition the dc-link current. One stage 30 is used to derive the motor average current and the other stage 14 is used for instantaneous dc-link over current protection.
The requirement of two amplifier and filter stages for average current sensing and instantaneous dc-link over current protection can be expensive in cost-sensitive applications, such as but not restricted to electro-hydraulic power steering systems.